(1) Field of the Invention
This invention relates to a process for manufacturing a double oriented electrical steel sheet including recrystallized grains whose easy axis &lt;001&gt; of magnetization is oriented both in the longitudinal orientation and in the direction vertical thereto, together with the rolled surfaces exhibiting {100} planes (those crystallographic orientations can be represented as {100} &lt;001&gt; in the Miller indices).
(2) Description of the Related Art
Since the double oriented electrical steel sheet has excellent magnetic properties in two different directions, because of its easy axis (&lt;001&gt; axis) in the rolled direction and in the direction vertical thereto, it can be more advantageously used for a magnetic core material of a specific apparatus, e.g., a large-scale rotating machine, where the magnetic flux flows in two different directions in comparison with a grain oriented electrical steel sheet which exhibits excellent magnetic properties in only one rolled direction. Non-oriented magnetic steel sheet, whose easy axis is not densely accumulated, are generally used for a small stationary machine or installation. The use of double oriented electrical steel sheet, however makes it possible to miniaturize the machine with an increased efficienty.
The double oriented electrical steel sheet, which has excellent magnetic properties as described above, has long been expected to be put into mass production, but the general use of such a type of sheet as an industrial product is still limited at present.
The following two methods in the prior art have been proposed for manufacturing a double oriented electrical steel sheet:
A method wherein an initial steel sheet is annealed at a high temperature in an atmosphere containing a polar gas, e.g., hydrogen sulfide, to secondarily recrystallize out {100} &lt;001&gt; oriented grains with the aid of surface energy, as described in Japanese Examined Patent Publication No.37-7110. This method is inadequate for mass production, however, because it requires a very accurate control of the surface energy of the sheet.
An other method wherein a steel sheet is cold-rolled in one direction and then cold-rolled in a direction vertical thereto, i.e., a cross cold r olling method", as described in Japanese Examined Patent Publication No. 35-2657. The magnetic flux density (B.sub.8) of the products obtained by this method is not more than 1.85 Tesla, and accordingly, a significant improvement of the magnetic properties can not be obtained in spite of the complicated manufacturing process, which in turn requires an increased cost. The double oriented electrical steel sheet obtained by this method is not preferable to the conventional grain oriented electrical steel sheet.
The magnetic flux density (B.sub.8) of the grain oriented electrical steel sheet has steadily improved, since the techniques disclosed in Japanese Examined Patent Publication No.40-15644 and Japanese Examined Patent Publication No. 51-13469 were disclosed. At present, the magnetic flux density (B.sub.8) of the commercially available products is as high as 1.92 T.
An improved method has been proposed to enhance the magnetic properties in a double oriented electrical steel sheet, as disclosed in Japanese Examined Patent Publication No. 35-17208 and Japanese Examined Patent Publication No. 38-8213. Nevertheless, the magnetic flux density of the resulting products has not been made higher than that of the grain oriented electrical sheet.